1. Field of the Invention
The present invention relates to decoding of trellis-encoded signals and more particularly to systems and methods of symbol correction in predictive decision feedback equalization architectures.
2. Description of Related Art
Since the adoption of the Advanced Television Systems Committee (“ATSC”) digital television (“DTV”) standard in 1996, there has been an ongoing effort to improve the design of receivers built for the ATSC DTV signal as described in the ATSC standard A/54 (see U.S. patent application publication 20050163209 for . . . ). Designers face major obstacles in designing receivers that might achieve good reception is the presence of multipath interference in the channel. Multipath interference affects the ability of the receiver to correctly decode transmitted symbols. Therefore, designers often add equalizers to receivers in order to cancel the effects of multipath interference and thereby improve signal reception.
Referring to FIG. 1, in the ATSC DTV transmission system, data is transmitted in frames 10. Each frame 10 is composed of 2 fields 11 and 12, each field 11 and 12 having 313 segments, and each segment having 832 symbols. The first four symbols in each segment are segment sync symbols 13 having the sequence [+5, −5, −5, +5]. The first segment in each field is a field sync segment 14 and 15.
Referring to figure shown in more detail in FIG. 2, field sync 20 comprises segment sync 21, a 511 symbol pseudo noise (PN511) sequence 22, a 63 symbol pseudo noise (PN63) sequence 23, a second PN63 sequence 24, a third PN63 sequence 25, and a 128 symbol sequence 26 composed of various mode, reserved, and precode symbols. The four PN sequences 22-25 are composed of symbols from the set {+5, −5}. In alternate fields, the three PN63 sequences 23-25 are the same. In the remaining fields, the first PN63 23 and third PN63 25 are the same while the second PN63 24 is inverted.
As shown in FIG. 3, subsequent 312 segments 30 of the field 11 and 12 (referred to as data segments) are structured such that 828 symbols 32 following the four segment sync symbols 31 are trellis encoded by a 12 phase trellis encoder described in detail in ATSC standard A/54. This results in 8 level symbols derived from the alphabet {−7 −5 −3 −1 +1 +3 +5 +7}.
Consider now an 8T-VSB transmitter such as is illustrated in FIG. 4. Input data 40 is first randomized 41, Reed-Solomon byte wise encoded 42, and then byte interleaved 43. Next the data is trellis encoded by a 12-phase trellis encoder 44. A multiplexer 45 adds the segment sync symbols and the field sync symbols to the trellis coded data at the appropriate times in the frame. Then, a pilot is inserted 46 by adding a DC level to the baseband signal and a modulator 47 modulates the resulting symbols to IF. Finally a RF upconverter 48 converts the signal for RF transmission as a vestigial sideband (VSB) signal at a symbol rate of 10.76 MHz.
Now consider a baseband model of the transmission channel fed by the above transmitter. The transmitted signal has a root raised cosine spectrum with a nominal bandwidth of 5.38 MHz and an excess bandwidth of 11.5% centered at one fourth of the symbol rate (i.e., 2.69 MHz). Thus the transmitted pulse shape q(t) is complex and given byq(t)=ejπFst/2qRRC(t),where Fs is the symbol frequency, and qRRC(t) is a real square root raised cosine pulse with an excess bandwidth of 11.5% of the channel. The pulse q(t) is referred to as the “complex root raised cosine pulse”. For the 8T-VSB system, the transmitter pulse shape q(t) and the receiver matched filter pulse shape q*(−t) are identical since q(t) is conjugate-symmetric. Thus the raised cosine pulse p(t), referred to as the “complex raised cosine pulse”, is given byp(t)=q(t)*q*(−t)where * denotes convolution, and * denotes complex conjugation. The transmitted baseband signal of data rate 1/T symbols/sec can be represented as:
            s      ⁡              (        t        )              =                  ∑        k                                      ⁢                          ⁢                        I          k                ⁢                  q          ⁡                      (                          t              -              kT                        )                                ,where {IkεA≡{a1, . . . , a8}⊂R1} is the transmitted data sequence, which is a discrete 8-ary sequence taking values on the real 8-ary alphabet A. The physical channel between the transmitter and receiver is denoted c(t) and can be described by:
      c    ⁡          (      t      )        =            ∑              k        =                  -                      L            ha                                      L        hc              ⁢                  c        k            ⁢              δ        ⁡                  (                      t            -                          τ              k                                )                    
where {ck(τ)}⊂C1, Lha and Lhc are the number of maximum anti-casual and casual multipath delays, τk multipath delay, and δ(t) is the Dirac delta function. Hence, the overall channel impulse response is:
      h    ⁡          (      t      )        =                    p        ⁡                  (          t          )                    *              c        ⁡                  (          t          )                      =                  ∑                  -                      L            ha                                    L          hc                    ⁢                        c          k                ⁢                  p          ⁡                      (                          t              -                              τ                k                                      )                              
In the 8T-VSB receiver block diagram depicted in FIG. 5, tuner 50 and IF filter 51 demodulate an RF signal to baseband. Timing and synchronization recovery is performed 52 and any NTSC interference is rejected 53. Data is then equalized 54 and sent through a phase tracker 55 and trellis decoded 56, de-interleaved 57, Reed-Solomon decoded 58, and finally de-randomized 59. The matched filter output y(t) in the receiver prior to equalization is:
                    y        ⁡                  (          t          )                    =                                    (                                          ∑                k                                                                              ⁢                                                          ⁢                              δ                ⁡                                  (                                      t                    -                    kT                                    )                                                      )                    *                      h            ⁡                          (              t              )                                      +                  v          ⁡                      (            t            )                                ,                  ⁢    where              v      ⁡              (        t        )              =                  η        ⁡                  (          t          )                    *                        q          *                ⁡                  (                      -            t                    )                    denotes the complex (colored) noise process after the pulse matched filter, with η(t) being a zero-mean white Gaussian noise process with spectral density σn2 per real and imaginary part. Sampling the matched filter output y(t) at the symbol rate produces the discrete time representation of the overall communication system according to the following equation:y[n]≡y(t)|t=nt=ΣIkh[n−k]+v[n]
Broadcast television channels are a relatively severe multipath environment due to a variety of conditions encountered in the channel and at the receiver. Only 728 symbols of a VSB field sync segment are known a priori and can be used as a training sequence for an adaptive equalizer. The channel is not known a priori, so the equalizer in the receiver must be able to adaptively identify and combat the various channel conditions. Since multipath signals in the broadcast channel may arrive many symbols after the main signal, the decision feedback equalizer (DFE) is invariably used in 8T-VSB applications. Another DFE structure that is well known is the noise predictive decision feedback equalizer (pDFE). Although both DFEs and pDFEs are good at combating multipath channels, both have the problem of error propagation. Error propagation occurs when there are errors in the feedback path. This, in turn, feeds erroneous data into the decision device resulting in incorrect symbol decisions. For 8T-VSB applications, the most commonly used decision device is the Viterbi Decoder. Therefore it is important to mitigate the effects of error propagation.
Since the 8T-VSB symbols are convolutionally coded, they may be decoded in the receiver with a Viterbi decoder [ATSC Standard A/54, U.S. Pat. No. 5,600,677, U.S. Pat. No. 5,583,889]. The Viterbi Algorithm (VA) for maximum likelihood sequence estimation of transmitted symbols corrupted by white noise is very well known (see “The Viterbi Algorithm”, G. D. Forney, Jr., Proc. IEEE, vol. 61, pp. 268-278, March 1973, “Digital Communications—Fundamentals and Applications”, Bernard Sklar, Prentice-Hall, 1988). The decoder may equivalently provide estimates of the encoded bit pairs or estimates of the mapped 8 level symbols, the later being utilized in the context of an equalizer. As is well known, the VA requires a path history memory for each state and involves add, compare, select operations based on trellis path metrics determined from sums of Euclidean distance branch metrics. As time advances, the most likely trellis paths (as indicated by the lowest path metrics) into each state of the trellis are saved, the rest are discarded. If the decoding algorithm searches back sufficiently deep in the trellis path memory, the result of discarding less likely paths—leaving only survivor paths—is a single surviving branch which defines the most likely symbol (hard symbol decision) at that prior point in time. At shallower path memory trace back depths (closer to the present time), there is a higher likelihood of multiple surviving branches with symbol probabilities proportional to the corresponding path metrics.